A known approach in controlling a Voltage Source Converter (VSC) based High Voltage Direct Current (HVDC) terminal for two-terminal, or point-to-point operation can include a three layer hierarchical control system with the following layers:
A firing control layer that generates firing pulses to the individual valves in the converter bridges. A common method for firing control is Pulse Width Modulation (PWM) using a reference sine wave. This enables control of the AC voltage magnitude and phase angle of the converter AC terminal to specified reference values.
A Converter unit control layer that generates the reference values for the converter AC voltage magnitude and phase angle to control the current through the converter to specified reference values generated by the system control. Limiter action is introduced to limit the voltage magnitude of the converter AC voltage to an allowed operating range.
A system control layer when applied in a point to point connection can have one terminal set to control DC voltage and the other terminal set to control DC current or an AC bus power reference. Additionally another controller can change the converter AC voltage magnitude reference value to control either the AC reactive power injection by the converter station Point of Common Coupling (PCC) or the AC voltage magnitude at PCC. The system control layer can be implemented as an array of cascaded PI-controllers with various internal limiters. Limiter action is also introduced on the output of the system control to limit the converter AC and/or DC current to an allowed operating range.
This three layer control system or modified forms thereof can be referred to as “VSC controller” or “local station control” in the following.
In Multi-Terminal HVDC (MTDC) systems with at least three terminals or converter stations interconnected by a DC grid known system controls as described above can be inadequate since the control is incapable of maintaining stability and balancing the load among remaining converters following severe disturbances in the MTDC system including failure of one of the converter stations and ensuing protective action. For example, failure of the DC voltage controlling terminal often causes the control systems of the remaining converters to go into limitation, and while the MTDC system can still operate, the controllability of the power flow is lost.
As a remedy, so-called droop control schemes introducing a droop constant k in a droop characteristic between the actual DC power PDC and DC voltage VDC in the system control layer have been devised in order to achieve stability of the individual VSC control also in multi-terminal operation. In such droop control schemes the local station controller no longer enforces perfect reference tracking, e.g., a perfect match of PDC to Pref in steady-state. This is advantageous since it ensures stability of the local station controllers also in multi-terminal operation. An exemplary droop characteristic reads PDC=Pref+k*(VDC−Vref), illustrating that PDC is allowed to deviate from a reference power Pref by a stationary error dependent on the droop constant k. Hence, instead of individually setting the values of the set-points Pref strictly equal to scheduled power flows Psched and hoping for the best when it comes to the actual schedule deviations, e.g. in case of outage of a cable or converter station, the use of a droop scheme allows a distribution of the schedule deviations over several terminals. However, droop control does not provide any way of guarding against overload of individual DC cables. Furthermore, it is difficult to tune the droop control schemes to provide a satisfactory load redistribution following converter outages or islanding in DC grids of a larger size and/or complex topology.
Control systems inherently include control system limits implying e.g., that a DC voltage set-point does not exceed a maximum allowed DC voltage which in turn depends on the insulation capabilities of the DC lines. Controllers can become less reliable or robust when approaching their controller limits and/or even tend to saturate if expected to operate too far off an initially devised safe operating regime corresponding to some underlying physical constraints. Initial simulation results for MTDC systems have now shown that there is even a possibility for harmful interaction between limiters in the system control layers and limiters in the converter unit control layers. While the behaviour of the P- and PI controllers in a control system is straightforward to analyse and tune for example using eigenvalue or modal analysis techniques, the behaviour of their associated limiters is far less explored.
A paper by Lu, W and Ooi, B. T. entitled “DC Voltage Limit Compliance in Voltage-Source Converter based Multi-Terminal HVDC”, IEEE PES General Meeting 2005, pp. 1322-1327, discloses a way of controlling steady-state DC bus voltages by pre-calculating and activating reference set-points that can be adapted to accidental loss of any one converter. In situations where the dc transmission distances can be long and the converter powers can be large, the set-points can be re-adjusted in an iterative manner until the voltage margins can be satisfied. This problem is formulated as optimization of a cost function, which is then solved by the Lagrange Multiplier method, subject to both voltage and power constraints.
The patent application WO2010/086071 is directed to a High Voltage Direct Current (HVDC) link with Voltage Source Converters (VSC) and interconnecting two power systems. A model-predictive control with a receding horizon policy is employed in the outer loop of a two-loop or two-layer control scheme for the HVDC link. While optimum converter current reference values can be passed from the outer to the inner loop, the control parameters of the inner loop remain unchanged. The two-loop control scheme takes advantage of the difference in speed of the dynamics of the various system variables of the HVDC link and the interconnected power systems. Model-based prediction representative of the interconnected power systems' behaviour allows comparing the future effect of different control inputs applied within the control scheme, while taking into account any physical, safety and operating constraints.
The patent application WO2012/044369 discloses a central application for coordinated control of Multi-Terminal HVDC Systems, simultaneously generating set-points for a VSC HVDC grid by solving an Optimum Power Flow (OPF) problem. The proposed solution disregards any constraints or limits, the latter have to be verified subsequently on the working system.